Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Digital electronic devices such as mobile phones and tablet computers can employ various types of optical devices such as image sensing modules and display modules. In such optical devices, optical elements such as lenses or diffraction gratings can be integrated with light-emitting elements such as LDs (laser diodes) and LEDs (light-emitting diodes), or light-receiving elements such as PDs (photodiodes) and CCDs (charge-coupled devices). In particular, as the size (e.g., thickness) of portable electronic devices is gradually decreasing, there are ever-increasing demands for further reduction in the size of optical elements installed in such devices.
Conventionally, optical devices may be fabricated by assembling optical elements and their corresponding light-emitting or light-receiving elements, which may be produced separately from the optical elements. In particular, optical devices such as lenses may be fabricated by molding technology such as injection molding. Further, light-emitting or light-receiving elements may be fabricated in the form of a wafer by semiconductor fabrication processes and diced out in the form of chips from the wafer. Such chips may then be individually assembled with corresponding optical elements. With this approach, however, it may be difficult to reduce the size of the optical devices. Also, it may take a considerable time for the assembly process, resulting in low production efficiency and high costs.
To resolve various problems, another fabrication process has been proposed in the related art, where multiple optical elements may be fabricated in the form of a wafer and mounted together at the wafer level with a corresponding number of light-receiving (or light-emitting) elements fabricated by semiconductor processes. In this process, since both optical elements and its corresponding light-receiving elements are fabricated in the form of a wafer, the assembling of these two elements can be performed in a more efficient manner. Also, since the assembling of the corresponding elements is conducted at the wafer-level, an optical device with a reduced size can be produced.
However, in such wafer-level optical device fabrication, to ensure precise optical performance, it is important to align the positions of the corresponding elements formed in different wafers. Specifically, when multiple wafers are stacked and assembled with each other, the optical axes of optical elements such as lenses should be aligned with the centers of light-receiving elements over the entire wafer planes. For example, if the wafers have a size of 8 inches (which is equivalent to about 200 mm), the alignment process should be performed within a tolerance of about 1 μm. Further, in case the wafers are made of different materials with different thermal expansion coefficients, it may be difficult to ensure positional precision in aligning the corresponding elements over the entire wafer planes, due to various temperature conditions during the fabrication process. In particular, since optical elements such as lenses may generally be fabricated using plastic materials, there may be a considerable disparity in thermal expansion coefficient relative to the silicon substrates on which light-receiving elements are formed.